1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly, to an optical disk apparatus which records/reproduces data on and from an optical disk having sawtooth wobbles (STW) formed thereon.
2. Related Art
In an optical disk apparatus, wobbles of a track of an optical disk are detected, and the thus-detected wobbles are used for detecting an address position and controlling a rotation speed. In connection with those wobbles, there have already been proposed a technique for wobbling a track through use of a cosine wave having a fundamental frequency fwob (22 kHz) and a technique for forming a sawtooth wobble SWT by means of adding or subtracting a sine wave whose frequency is double that of the cosine wave.
FIG. 10 shows a modulation scheme using a sawtooth wobble STW. Portions of a fundamental cosine wave; i.e., cos (2π·fwob·t), are replaced with a STW that is obtained by means of adding or subtracting a sine wave; i.e., a·sin {2π·(2·fwob)·t}, to or from the cosine wave. Here, a=0.25. +cos (2π·fwob·t)+a·sin{2·(2·fwob)·t}, which is obtained by adding the sine wave to the cosine wave, represents a digital value of “1.” Further, cos (2π·fwob·t)−a·sin{2π·(2·fwob)·t}, which is obtained by subtracting the sinewave from the cosine wave, represents a digital value of “0.” The STW obtained by adding a sine wave to a cosine wave can be represented as “+STW,” “right slope STW,” “mild up slope right STW,” or the like. A signal obtained by subtracting a sine wave from a cosine wave can be represented as a “−STW,” “left slope STW,” “mild down slope left STW,” or the like. In the present patent application, an STW obtained as a result of addition of a sine wave is denoted as “+STW,” and an STW obtained as a result of subtraction of a sine wave is denoted as “−STW.” Information about an optical disk, such as a disk size, a disk structure (a single layer or a multilayer), and optimum laser power, can be recorded on an optical disk by means of inserting a +STW or a −STW.
As mentioned above, use of a +STW and a −STW enables embedding of various information items into a wobble signal. Extraction of a wobble signal from a signal reflected from an optical disk and reliable identification of a +STW from a −STW are required.